1. Field of the Invention
The present invention relates generally to visual displays and more particularly to headgear-mounted displays.
2. Description of the Related Art
A variety of visual-display systems have been proposed for presenting an information display to a system user while that user is concentrating on a visual scene which is critical to the user's immediate task, e.g., driving a high-speed motor vehicle, piloting an aircraft or operating a mobile, motion-picture camera. These systems are generally positioned on the user's head and are often integrated into a headgear which is associated with the user's task, e.g., eyeglasses or a helmet. The systems are typically referred to as head-up displays (HUD) because they facilitate concentration on a visual scene that is generally in front of the user.
Visual-display systems have included a variety of image generators. For example. U.S. Pat. No. 3,833,300 combines an image generator in the form of a cathode-ray tube with a helmet visor which is configured in the shape of a parabola. An optical fiber conducts the image of the cathode-ray tube to the focal point of the parabola. From the focal point, the image is directed at the visor. The visor preferably carries a reflective coating to enhance a reflection of the image to the wearer of the helmet.
A parabolically-shaped, helmet visor is also described in U.S. Pat. No. Re. 28,847. An image generator is positioned at the focal point of the parabola and directed at the visor surface. A coating is applied to the visor surface to cause it to be a combining surface, i.e., it is partially reflective and partially transparent. In another embodiment, the visor is altered to place the parabolic focal point within the forehead of the wearer of the helmet and a mirror is added to reflect the image from the image generator to the visor. The mirror is positioned so that a virtual image appears to be located at the focal point.
Another exemplary use of a cathode-ray tube as an image generator is found in U.S. Pat. No. 4,761,056. From a cathode-ray tube, an image is reflected from a first parabolic mirror and then directed along an optical path which is defined by a pair of fold mirrors. The optical path intersects a second parabolic mirror which is in the field of view of the system's user. In one embodiment the first and second parabolic mirrors are segments of a helmet visor. The parabolic mirrors are preferably formed by partially reflective coatings on the visor.
U.S. Pat. No. 5,189,512 describes yet another exemplary display system in which a cathode-ray tube functions as an image generator. An image, which is formed by the cathode-ray tube, is directed along an optical path with a pair of fully-silvered mirrors. Positioned between the mirrors is a double convex lens. The second mirror is placed in front of one eye (the "display eye") of a user of the system. The spacing between the lens and the cathode-ray tube is adjusted to position a virtual image at a distance, e.g., 3 meters, sufficient to avoid the need for extensive refocusing when attention is diverted away from the user's immediate task. Preferably, an occlusion device in the form of an opaque element is positioned in front of the user's other eye (the "non-display eye"). This element is arranged in a shape which approximates the shape of the perceived image in the display eye. The occlusion device is intended to prevent "binocular rivalry" in which the brain's perception areas become confused by conflicting visual information.
Other visual-display systems have employed liquid-crystal displays as a display generator. In U.S. Pat. No. 4,181,405, each visual element, e.g., an alpha-numeric symbol, of an information display is generated by a reflective liquid-crystal display. A helmet visor is configured with a plurality of convex lenses and each liquid-crystal display is separated from a different lens by a space that is generally equal to or less than the focal length of the respective lens.
Another exemplary system is described in U.S. Pat. No. 4,869,575, which positions a reflective liquid-crystal display in association with a body of optically-clear material. The body has a prism portion and a collimating lens portion. In one embodiment, the prism portion forms a planar surface and a pair of reflecting surfaces and the lens portion forms a spherical convex lens. The liquid-crystal display is positioned against the planar surface and the body is mounted to the frame of a pair of eyeglasses with the lens positioned proximate to one glass of the eyeglasses. In operation, the lens receives the image from the reflecting surfaces and forms a virtual image at optical infinity. To view the virtual image, a user looks slightly above the normal sight line through the eyeglasses.
In still another exemplary system, U.S. Pat. No. 5,162,828 mounts a transmissive liquid-crystal display in various headgears, e.g., ski goggles, scuba dive masks and sunglasses. The liquid-crystal display is illuminated by ambient light that reaches the display through a diffusing filter. Alternatively, the ambient light is replaced by light from an incandescent source which is reflected from a parabolic mirror. In one embodiment, a partially-reflective mirror is positioned above or to the side of the user. A lens is placed between the liquid-crystal display and the mirror to provide an apparent optical distance which is selected to correspond to the visual distance that is associated with the immediate task of the wearer.
Although these exemplary display systems may adequately present an information display to a system user when the user's immediate task is performed in a benign environment (e.g., a motion-picture studio), they generally are not suitable for tasks (e.g., operating a racing vehicle) that are performed in the presence of harsh conditions (e.g., high wind, vibration and shock and strong ambient light). Under these conditions, a display system attached to an eyeglass frame (see U.S. Pat. No. 5,162,828) would not survive. A display system arranged within a helmet (see U.S. Pat. No. Re. 28,847) can become a lethal object to the system user and one whose center of mass is located at one side of a user's head (see U.S. Pat. No. 4,761,056) can induce dangerous twisting forces on the head. Such harsh conditions demand a system which can present an image that is visible in the presence of strong ambient light and which is configured to protect a user of the system from injury.